Rheological Behavior of (Short) Carbon Fiber/ Thermoplastic Composites. Part II: The Influence of Matrix Type

Abstract

This study is the second of a series of two papers on the rheological properties of VGCF/thermoplastic composites, its aim being to compare the performance of different thermoplastic matrices when reinforced with sub-micron Vapor Grown Carbon Fibers (VGCF). For this purpose, two polymers (polypropylene, PP, and polycarbonate, PC) were used to produce composites with different levels of fiber incorporation. The highest content VGCF composites were produced, in pellet form, using a co-rotating twin-screw extruder. These were subsequently diluted during the injection molding stage to obtain composites with different levels of fiber content. The rheological characterization was performed, using the latter materials, by means of both capillary and rotational rheometries, results being presented and discussed in terms of reinforcing capability and its susceptibility to temperature, for the relevant functions in both shear (steady and oscillatory) and extensional flows. The results show that VGCF have a higher reinforcing capability, both rheological and mechanical, when incorporated in the PP matrix. This better performance can be due either to better adhesion between the fibers and PP in comparison to PC, or to a higher degree of melt penetration of non-wetted fiber rich areas and lower fiber length degradation, owing to the lower viscosity of PP in the extrusion shear rate range.